Signal superimposition

ABSTRACT

A method and apparatus are provided in which control data for a generator system is multiplexed onto a synchronization signal.

FIELD OF THE INVENTION

The present invention relates to a method of transmitting two signalsover a single transmission line, and in particular to a method oftransmitting a frequency reference signal and a control command signalfor a generator command unit over a single transmission line, and to anapparatus utilising such a method.

BACKGROUND TO THE INVENTION

As electronic equipment and/or systems controlled by such equipmentbecome ever more complex so the number of signals required to betransmitted between individual pieces of equipment and/or subsystemsincreases. There is therefore a corresponding increase in the number oftransmission lines, or cables, that are required to be provided betweenindividual items of equipment. In certain applications where physicalspace is limited, or where it is desirable to minimise the weight ofphysical transmission lines, or where new systems are being retrofittedinto existing installations it becomes impracticable or impossible toprovide a wire for each individual signal.

One such area where this conflict arises is on aircraft. Most aircrafthave a number of onboard electrical generators that are driven by theaircraft engines to supply the power for the aircraft's electronic andelectrical systems. Generally, each generator is controlled by anassociated electronic generator control unit (GCU) that ensures thepower supply provided by the generator is within certain predeterminedparameters e.g. frequency and voltage, that are appropriate for theelectrical equipment to be supplied. However, there is clearly arequirement to provide a power supply to the aircraft electrical systemswhen the aircraft is on the ground and the engines are not in operationor are in a start up or shut down mode. For example, it is oftendesirable to operate the air conditioning units and lighting on theaircraft when it is on the ground to allow simple maintenance andcleaning operations to be completed, or equally to enable the variousflight systems to be operated, such as navigation computers, prior tothe departure of the aircraft. It is therefore necessary to provide anauxiliary power supply. The auxiliary power supply may either be aportable generator, for example a diesel powered generator, that isconnected to the aircraft's electrical system, or may be an auxiliarypower unit mounted on the aircraft itself.

In either case, when switching between the aircraft's power supply andthe auxiliary power supply it is undesirable to have any interruption inthe power supply to the electrical systems. The seamless switch from onepower supply to another is referred to as a “no break power transfer”(NBPT) and it is accomplished by very briefly having both power suppliesconnected to the aircraft systems at the same time.

Successfully providing NBPT requires careful synchronising of theauxiliary power supply and the aircraft generators to avoid any suddenunequal loading of the electrical systems. This is accomplished byproviding a fixed frequency reference signal from a frequency referenceunit to the aircraft's generator control units and also providingcommand signals that are transmitted between the auxiliary power supplycontrol unit and the aircraft generator control units. The frequencyreference signal and the command signals have in the past been providedover separate transmission lines or cables. It is not always practicableor possible to provide individual signal lines, especially whereequipment is being retrofitted.

It is therefore desirable to provide a method of transmitting more thanone signal using the same transmission line.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is provided amethod of transmitting data over a single transmission path, the methodcomprising the steps of providing a first signal, and modulating saidfirst signal with a second signal, said second signal having a higherfrequency content than said first signal.

Preferably the first signal comprises first periods of a first signallevel separated by second periods of a second signal level differentfrom the first signal level.

In a preferred embodiment there is provided a method of transmitting twosignals over a single transmission path, the method comprising:providing a first signal comprising a series of pulses, each pulsehaving a first pulse duration and transmitting said first signal overthe transmission path; and providing a second signal comprising a seriesof pulses, each pulse having a second pulse duration, the second pulseduration being less than the first pulse duration, and transmitting saidsecond signal over the transmission line, whereby said second signal istransmitted within said first pulse duration.

The first signal may be a timing reference signal having a fixedfrequency, such that the signal level alternates between the two signallevels. The second signal may be a data signal and may be transmittedonly in between each transition of the first signal.

The second signal is preferably transmitted by imposing it on the firstsignal during the first periods thereof.

Preferably guard periods are provided either side of the second signalsuch that signal transitions belonging to the first signal can bedistinguished from signal transitions belonging to the second signal.

The second signal may in fact be comprised of more than one signal, eachpart of the second signal being transmitted in between differenttransitions of the first signal.

The first signal may alternatively be a data signal provided that asecond signal is not transmitted for a predetermined period of timespanning the end of one pulse of the first signal and the beginning ofthe consecutive pulse of the first signal such that the state of thefirst signal at the beginning of each pulse can always be determined.

The first signal may be a frequency reference signal for thesynchronisation of a first and a second generator and the second signalmay comprise control signals transmitted between respective controlunits of a first and second generator.

According to a second aspect of the present invention there is provideda generator control system comprising generator controllers responsiveto a first signal path, wherein a first signal is transmitted on thesignal path, and a second signal is transmitted on the signal path, thesecond signal modulating the first signal and having a higher frequencycontent than the first signal.

According to a third aspect of the present invention there is provided acontroller for a generator, wherein the controller has a first signalconnection and the controller is responsive to a synchronisation signalon the first signal connection and is responsive to command signalsmodulated onto the synchronisation signal and having a shorterinter-pulse period than the synchronisation signal.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the present invention will further be described, by wayof example, with reference to the accompanying drawings, in which:

FIG. 1 is a schematic representation of an aircraft generator system andauxiliary power supply;

FIG. 2 represents a frequency reference signal using the power supplysystem of FIG. 1;

FIG. 3 shows a generator control unit command signal used in the powersupply of FIG. 1 superimposed on the frequency reference signal of FIG.2; and

FIG. 4 shows the resulting communication signal for the power supplyunit of FIG. 1 in accordance with an embodiment of the presentinvention.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 is a schematic representation of an aircraft power supply 1connected to a power control unit 2. The aircraft power supply 1includes four generator control units 5 a, 5 b, 5 c, 5 d. Each generatorcontrol unit is connected to a respective electrical generator, althoughfor the sake of clarity only one generator 8 is illustrated. It will beappreciated that the number of generators and generator control unitswill vary from aircraft to aircraft and the system shown in FIG. 1 issimply a representative example. Each generator control unit 5 controlsthe power output by its respective generator, in particular thefrequency, phase and magnitude thereof. The power control unit 2comprises an auxiliary power control unit 3 connected to an auxiliarygenerator (not shown) and/or a ground power control unit 4 connected toone or more external ground generator sources (not shown). The groundpower control unit 4 is also connected to each of the generator controlunits 5 a, 5 b, 5 c, 5 d onboard the aircraft by means of acommunications cable 6. The communication cable 6 may be abi-directional RS485 data link, with a data transfer rate of 100 kb persecond, although other suitable data links may be used.

The generator control units 5 a-5 d each receive a frequency referencesignal. In general this signal is provided by a reference unit 7 whichmay be within the power control unit 2. This unit can also be used tosupply a reference signal to the auxiliary power control unit 3.However, where the ground based power unit is provided to initiateaircraft operation, the control unit associated with the ground basedgenerator may act as the master for the frequency reference signal. Insuch an arrangement the auxiliary power control unit 3 on the groundbased generator must supply both the frequency reference signal to theindividual generator control units 5 a-5 d and must also establish datacommunication with the controllers such that each controller can verifythat the system is operating correctly and can be appraised of a faultcondition if one arises.

The frequency reference signal is a 400 Hz square wave signal, as shownin FIG. 2.

The 400 Hz square wave frequency reference signal shown in FIG. 2provides the frequency reference signal for no break power transfer(NBPT) when the power supply is transferred between the generatorsonboard the aircraft and the auxiliary ground based generator of thepower control unit 2. For synchronisation purposes it is only necessaryto detect the leading edge of the frequency reference signal. Toeliminate any spurious triggers due to noise on the signal, filtering ofthe signal is used by the ground power control unit 4 and the generatorcontrol units 5. By using filtering, the leading edge of the frequencyreference signal can be identified by the signal being low for a certainlength of time, followed by the signal going high and remaining high fora minimum length of time. This is indicated on FIG. 2 by the timeinterval marked X, which denotes the portion that identifies thefrequency reference leading edge. It therefore follows that theremainder of the frequency reference signal when the signal is highconveys no further useful information.

To accomplish the synchronisation and the no-break power transferbetween the onboard power supply 1 and the power control unit 2, theground power control unit 4 and the generator control units 5 of theonboard power supply 1 exchange command signals. In the describedembodiment of the present invention the protocol for the command signalscomprises four word commands. The first word always has the same valueand is denoted as purely a start word. The last word, again always hasthe same value and simply indicates the end of the message. The middletwo words transfer the actual message information. Each transmitted wordtypically comprises 1 start bit, 8 data bits, 1 parity bit and 2 stopbits, giving a total of 12 bits per word. Therefore, the four wordcommand message comprises 48 bits. If the command message is thereforetransmitted at 40 Kb per second then each bit would have a duration ofapproximately 0.025 mS per bit. Thus, to transmit the four word, 48 bit,command message would require 1.2 mS. Referring back to FIG. 2, it canbe seen that it is therefore possible to transmit the four word commandmessage within each high phase (also known as a mark) of the frequencyreference signal.

The combined frequency reference signal and command message signal isshown in FIG. 3. The frequency reference information can be obtainedfrom the combined signal of FIG. 3 using the previously mentionedproperty that the leading edge is defined by the signal being low for acertain length of time followed by going high for a set period. Equally,the command message information can be extracted from the combinedsignal of FIG. 3 by sampling the signal at a suitable sampling rate, forexample at 1 MHz. A sampling rate of 1 MHz would provide 24 samples foreach data bit of the command message.

For a RS485 data link, the maximum data rate is 100 Kb per second. Thisequates to 0.010 mS per bit. Therefore each word of the command messagewould require 0.12 mS and the complete four word message would thusrequire 0.48 mS. Therefore there could be a maximum of two packets (offour words) in each high phase of the frequency reference signal.

Thus by combining the two signals a single transmission line can be usedwithout any loss of information.

In a further embodiment of the present invention, the time required todetect a rising edge or falling edge of the frequency reference signalmay be further reduced by suitable filtering such that data transmissionof the command message may be transmitted during both the high period ofthe frequency reference signal and the low period of the frequencyreference signal. In this way it is possible to arrange for a firstcommand message signal to be transmitted within the high period of thefrequency reference signal and a second, separate command signal to betransmitted during the low periods of the frequency reference signal.This is indicated in FIG. 4, where the frequency reference signal 10 isshown as a regular square wave, there being a data transmission window Yavailable during both the high periods of the frequency reference signaland the low periods of the frequency reference signal. In FIG. 4 thedata transmission window during the high periods of the frequencyreference signal are labelled as GAPCU unit signals, as issued by theground power unit 4 in the power control unit 2. The data transmissionwindows in the low periods of the frequency reference signal 10 arelabelled as the generator control unit data transmission windows for thetransmission of command signals from the generator control units 5 ofthe aircraft by supply system 1 to the power control system unit 2. Theresulting combined signal 12 is also illustrated on FIG. 4.

It will be appreciated by those skilled in the art that by using simpletime multiplexing techniques almost any number of higher rate signalsmay be combined with the frequency reference signals. The presentinvention enables the number of signal lines within an aircraft to bereduced, thereby reducing the weight of the wiring harness and/orenabling increasingly sophisticated control systems to be retrofitted toaircraft.

It will also be appreciated that the lower frequency signal, in theabove embodiments the frequency reference signal, does not have to be aperiodic signal. As long as a minimum time period is defined in whichany change in the signal level of the original signal may be detectedthen in fact the lower frequency signal may equally be a data signal.

Although the above embodiments have been described in relation to nobreak power transfer between a power supply unit on an aircraft and anauxiliary power supply unit, further embodiments encompassing theoverall concept of combining a first signal having a first frequencywith one or more second signals having higher frequencies to produce acombined signal that may be transmitted down a single transmission lineare equally applicable and are intended to be covered by the presentinvention.

1. A method of transmitting two signals over a single transmission pathfor synchronizing generators within an aircraft electrical system, themethod comprising the steps of transmitting a first of the two signalsand modulating the first signal with a second of the two signals, thesecond signal having a higher frequency content than the first signaland in which the second signal comprises a digital word, and the digitalword is sent in a period contained within a first or a second period ofthe first signal.
 2. A method as claimed in claim 1, wherein the firstsignal comprises first periods of a first signal level separated bysecond periods of a second signal level.
 3. A method as claimed in claim2, wherein the second signal comprises a plurality of pulses each pulsehaving a duration less than the first period of the first signal, andwherein the second signal is transmitted within the first period of thefirst pulse.
 4. A method as claimed in claim 2, in which thecommencement of the first period is marked by a signal transition, and aguard period in which the second signal is not transmitted is providedimmediately in advance of the signal transition.
 5. A method as claimedin claim 2, in which the commencement of the first period is marked by asignal transition, and a guard period in which the second signal is nottransmitted is provided immediately following the signal transition. 6.A method as claimed in claim 2, in which digital data is transmittedduring the second period.
 7. A method as claimed in claim 1, in whichthe first signal is a synchronisation signal.
 8. A method as claimed inclaim 1, wherein the second signal has a shorter inter-pulse period thanthe first signal.
 9. A method of synchronising generators within anavionics system having a plurality of generator controllers so as toperform no-break power transfer, the method comprising providing areference signal for frequency and/or phase synchronisation, thereference signal being transmitted along a transmission path, andtransmitting messages the between the generator controllers along thetransmission path such that the generator controllers can co-operate toperform the no-break power transfer, the messages being sent during amark or space period of the reference signal.
 10. A generator controlsystem comprising a plurality of generator controllers responsive to asignal path, wherein a first signal is transmitted on the signal path,and a second signal is transmitted on the signal path, the second signalmodulating the first signal and having a higher frequency content thanthe first signal, where the first signal is a reference signal forsynchronising a generator controller of the plurality and the secondsignal conveys commands or status data to the generator controller ofthe plurality.
 11. A generator control system as claimed in claim 10, inwhich the first signal comprises a plurality of first periods of a firstsignal level separated by second periods of a second signal level.
 12. Agenerator control system as claimed in claim 11, in which the secondsignal comprises a digital word, and the digital word is sent in aperiod contained within the first period of the first signal.
 13. Agenerator control system as claimed in claim 12, wherein thecommencement of the first period is marked by a signal transition of thefirst signal and a guard period is provided before and after thetransition in which the second signal is not transmitted.
 14. Agenerator control system as claimed in claim 13, in which thecontrollers synchronise with the first signal by identifying the signaltransition surrounded by the guard period.
 15. A generator controlsystem as claimed in claim 11, in which data is transmitted as a digitalword within the second period.
 16. A control system as claimed in claim10, wherein the second signal has a shorter inter-pulse period than thefirst signal.
 17. A device comprising a controller for a generator,wherein the controller has a signal connection, wherein the controlleris configured to obtain generator synchronization timing data from atiming reference signal on the signal connection, and wherein thecontroller is further configured to exchange with other generatorcontrollers command signals modulated onto the timing reference signaland having a shorter inter-pulse period than the timing referencesignal.
 18. A method of transmitting two signals over a singletransmission path, for synchronizing generators within an aircraftelectrical system, the method comprising: providing a first signalcomprising a series of first pulses, each of the first pulses having afirst pulse duration, and transmitting said first signal over thetransmission path; and providing a second signal comprising a series ofsecond pulses, each of the second pulses having a duration, each of thesecond pulse durations being less than the first pulse duration, andtransmitting said second signal over the transmission path, whereby saidsecond signal is transmitted within said first pulse duration.